This invention relates to an actuator for advancing a rod.
An actuator advancing a rod is used, for example, in a dispensing appliance for dispensing viscous materials such as mastic caulking material.
Viscous material, such as mastic caulking material, is commonly supplied in a cartridge having a discharge nozzle. The cartridge can be mounted in a dispensing appliance, often called a dispensing gun. An example of such a dispensing gun is described in British Patent GB 1555455. The gun has a plunger or push rod slidably mounted in a stock. The cartridge is mounted in a keep before the plunger which is advanced by means of a catch plate engaging the rod by means of a trigger lever to force a piston, inside one end of the cartridge, forward to urge the material from the nozzle at the other end. The trigger and catch plate are reset at the end of a dispensing stroke and the dispensing force can be reapplied. During dispensing, the plunger is held against the piston in the cartridge between trigger strokes by means of a locking plate.
By maintaining and reapplying a constant force on the trigger, it is found that it is possible to exert a high degree of control over the rate of discharge of mastic material. The gun is entirely acceptable as a dispensing tool for viscous material in many situations. However, it is found that the body of the cartridge expands radially under a dispensing force. Similarly, if any pockets of gas are trapped in the cartridge they will compress during discharge of the material and expand once the dispensing force is removed. As the plunger is maintained by the locking plate substantially in the position at which the dispensing force was removed from the trigger, the contracting cartridge and any expanding pocket of gas tend to cause continued dispensing of material when it is not required unless the locking plate is released manually to retreat the rod. It is sometimes desirable to be able halt the discharge immediately as the hand releases the trigger.
To address this European Patent Application EP-B-0448375 discloses a pressure relief device whereby the rod is automatically retreated at the end of a trigger stroke (that is when force is released from the trigger) by a small amount to prevent the mastic from continuing to be dispensed. The mechanism works well as a modification of a conventional type of dispensing gun in GB 1555455 for certain situations.
Another development in actuators for advancing a rod can be found in U.S. Pat. No. 5,370,282 in which the inevitable wear from which the catch plate suffers can be compensated for. This is a different approach to the typical actuation as the catch plate is now biased to an ‘on’ or engaged relationship with the rod and is released by movement of a device that transmits a force applied to the locking plate to release the catch plate so that the rod can be retreated manually at the end of a trigger stroke.
Both the pressure relief concept and the wear compensating concept have their benefits in, for example, mastic guns, but it has not proved possible to combine both benefits in one actuator as they are implemented in conflicting ways.